Conventionally, a method of reducing an end portion of a pipe and the like in diameter is referred to as a spinning work.
Japanese Patent Unexamined Publication No. 11(1999)-147138 discloses a method (spinning work) of forming a pipe 4 (workpiece) having a circular cross sectional shape. In this first related art, a roller 28 of a forming apparatus is compressed against a circumferential end portion of a fixed pipe 4. Simultaneously with this, the roller 28 is revolved and put into the spinning work so as to form a narrow neck 4c at the end portion of the pipe 4. During the spinning work, a revolution axis X5 of the roller 28 is eccentric from a longitudinal center line X4 of the pipe 4, to thereby form the narrow neck 4c that is eccentric with respect to the longitudinal center line X4 of the pipe 4.
For carrying out the spinning work, the forming apparatus having the roller 28 is needed to be so constructed that the roller 28 is also movable radially with respect to the revolution axis X5 during its revolution. In the first related art, the forming apparatus has a bracket 27 for supporting the roller 28. The bracket 27 is provided with a first taper surface 27a. Moreover, the forming apparatus has a ring plate 26 which is provided with a second taper surface 26a. The first and second taper surfaces 27a and 26a abut against each other. The ring plate 26 is moved forward and backward relative to the pipe 4, to thereby allow the roller 28 to move radially with respect to the revolution axis X5.
Similarly to the above first related art, Japanese Patent Unexamined Publication No. 11(1999)-179455 discloses a method of forming a narrow neck 44 (reduced diameter portion) of a pipe 42 which is fixed. In this second related art, a roller 27 of a forming apparatus 10 is compressed against an external circumference at an end portion of the pipe 42, and is revolved during the spinning work for forming the narrow neck 44 with a reduced diameter.
In the forming apparatus 10 according to the second related art, a support shaft 28 for supporting a roller 27 is mounted on a spinning portion 24 via a slide lock plate 26. A plate cam 29 rotating with the slide lock plate 26 during the spinning work allows the support shaft 28 to move in a radial direction of the spinning portion 24.
A rod member 19 of a main shaft 18 penetrates through an axial center of the spinning portion 24. A pivot converting portion 19A converts an axial displacement of the rod member 19 into a rotation relative to the spinning portion 24. The plate cam 29 is driven by the pivot converting portion 19A.
A rotation axis 48 of the spinning portion 24 is deviated from a longitudinal center line 47 of the pipe 42, to thereby form the narrow neck 44 that is eccentric with respect to the longitudinal center line 47 of the pipe 42.
The above first and second related arts make it possible to produce casings such as those for catalytic converters for vehicles with a small number of component parts in a short time.
Each of the above first and second related arts requires the pipe (workpiece) that has a xe2x80x9ccircularxe2x80x9d cross sectional shape. However, the casing for the catalytic converter for the vehicle has an overall configuration preferably as flat as possible so that the casing can be mounted below a floor of the vehicle. Therefore, a workpiece shaped into an xe2x80x9cellipticalxe2x80x9d pipe is needed for the casing, instead of the circular pipe.
However, according to the first and second related arts, spinning an elliptical pipe (workpiece), instead of the circular pipe (workpiece), cannot always produce an elliptical pipe (product) having a desired narrow neck.
More specifically, FIG. 9 shows an elliptical pipe 70 (workpiece) made of steel having the following dimensions: a wide diameter of 150 mm, a narrow diameter of 105 mm, and a plate thickness of 1.5 mm, with an end portion thereof cut perpendicular to an axial direction. The elliptical pipe 70 (workpiece) is fixed and then put into a spinning work by means of a roller in order to reduce the end portion in diameter to 70 mm. As a result, a gradual reduction in diameter is seen with a pair of slopes 71 (taper) having an intersection angle of 60 degrees (not shown). However, the slope 71 and the narrow neck have an abnormal deformation 72, that is, a wall surface partly rolled inward. As a result, the thus obtained narrow neck (reduced diameter portion) does not have a cross section shaped into a complete round.
It is therefore an object of the present invention to provide a method of shaping an elliptical pipe (product) through a spinning work.
It is another object of the present invention to make an elliptical pipe (workpiece) usable for shaping the elliptical pipe (product) having a narrow neck which has a cross section shaped into a complete round.
There is provided a pipe shaping method, according the present invention. The pipe shaping method shapes a workpiece in the form of an elliptical pipe having a cross sectional shape elongated in a predetermined elongating direction and extending longitudinally from a first end to a second end, into a form having a narrow neck at least at the first end of the workpiece. The pipe shaping method comprises the following steps of: cutting the first end of the workpiece into a shape having a projection located at a subsidiary vertex portion and a recess located at a principal vertex portion, the projection projecting in a longitudinal direction of the workpiece, the recess being recessed in the longitudinal direction of the workpiece; mounting the workpiece to a pipe holder; disposing a roller around the workpiece; and reducing a cross sectional size of the first end of the workpiece by applying a compressive force on the circumference of the first end of the workpiece by a relative revolving movement between the roller and the workpiece.